Objects of metal and other materials which are subjected to high stresses tend to fail by developing one or more propagating ductile fractures, which are commonly called free-running cracks. The range of objects which exhibit such propagating ductile fractures is too large for all of the objects to be mentioned specifically in this application, but it includes compressed gas cylinders, pipes and so on. One of the areas which presently appears to be facing the greatest potential problems due to propagating ductile fractures is that of pipes, and especially the relatively large diameter pipes which are used in pipelines.
Pipelines of a relatively large diameter that are formed by pipes of steel or other metal are playing increasingly important roles in the transportation of gases, such as natural gas, CO.sub.2, and other volatile fluids. For example, with the increase in domestic gas reserves and a corresponding increase in the use of natural gas, there will be a corresponding increase in the need for pipelines for transporting the gas at gathering pipelines and through long-range transmission lines.
Especially if flawed or damaged, and if they contain fluid at relatively high pressures, these types of pipes are susceptible to a propagating ductile fracture, or free-running crack, that will travel at velocities of 400-1200 feet per second. The probability of this type of fracture initiating is increased if the product in the pipeline is corrosive, either because of the gas itself or because of moisture, which is often contained in the gas. The probability is also increased in the case of steel pipes carrying natural gas containing large quantities of hydrogen sulfide, which has been known to create hydrogen embrittlement. Furthermore, any moisture in the gas and any CO.sub.2 which may be present will generate carbonic acid, which can also damage steel pipes. In addition, the problem of failure is significantly compounded in a cold climate. Moreover, a propagating ductile fracture or free-running crack can be caused by impact from an external force such as a trenching machine accidentally striking the pipeline, earthquakes, frost heave and the like.
The tendency for objects to fail by a propagating ductile fracture is generally due to the nature of the material of which the object is made and the manner in which the material is worked or shaped to form the object. For example, many pipes for pipelines are long-seam welded pipes made from a metal block which is elongated more in one direction than in another by being passed between pairs of rollers or similar means. Such a method produces an elongated plate which is then formed into a "U" configuration by bending it about its longitudinal axis. The U-shaped plate is further bent into a cylindrical configuration by bringing the sides of the "U" into abutment in an "O" and joining the sides along the length thereof, such as by welding, to form a substantially straight longitudinal seam. In such a pipe, there is considerably greater strength in the longitudinal direction than there is in the circumferential or hoop direction, which defines an axis of inherent weakness, and, as a result, the pipe is able to withstand greater stress in the longitudinal direction than in the hoop direction. Therefore, when such a pipe fails, it is the result of a hoop of indeterminate width being broken and the ends of the broken hoop being separated. Then, adjacent hoops are broken as the result of the failure of the first hoop, and a line of separating hoop ends moves longitudinally along the pipe to define a propagating ductile fracture.
In addition to the long-seam welded pipes just mentioned, ductile fractures have also been known to propagate in a direction parallel to the axis of the pipe in spiral-welded pipes, which are formed by coiling a sheet of steel into a spiral and welding the adjacent edges of the sheet.
Although there have been numerous proposals to limit ductile fracture propagation in objects and especially in pipelines, including the use of heavy walled pipes, cables, concrete abutments, valves and metal sleeves, they have all been less than completely satisfactory.
For example, U.S. Pat. No. 4,195,669 to Ives et al discloses arresting ductile fracture propagation by providing an encircling mass of material around the pipe at preselected intervals as a circumferential restraint. However, the steel collar, the steel cable windings and the reinforced concrete cast disclosed by Ives et al are all quite heavy and difficult to handle and install. In addition, the crevice between the encircling masses and the pipe, and the crevices between adjacent windings in the case of steel cables, are subject to the ingress of dirt and moisture and the resultant crevice corrosion, which weakens the pipes. Moreover, the corrosion attacks the encircling masses themselves, especially in the case of cables. Furthermore, in metals having higher tensile strengths, the effects of corrosion are accelerated.
U.S. Pat. Nos. 4,148,127 and 4,224,966 to Somerville disclose a method of applying a band-type crack arrestor over the outer diameter of a pipe and applying radial force to the inner diameter of the pipe to engage the arrestor in a tight fit. The disclosed crack arrestors are preferably bands or rings of the same metal as the pipeline. Therefore, they suffer from the same great weight and crevice corrosion problems as the encircling masses of the Ives et al patent. In addition, the tight fit of the pipe with the edges of the crank arrestors tends to damage any underlying corrosion protection layers and to cause stress concentrations in the pipe where the crack arrestor edges engage it. Of course, stress concentrations can lead to premature failure of the pipe. The Somerville patents also disclose that the bands can be fiberglass, but state nothing more about fiberglass bands. If the band is a typical molded ring of randomly oriented glass fibers, the thickness required to stop a crack would be such as to make the band very bulky and unwieldy.
U.S. Pat. No. 3,870,350 to Loncaric discloses a pipe having zones of increased crack resistance due to cylindrical steel members surrounding the pipe, to which they are at least partially welded. The steel members are heavy and, thus, difficult to handle and install, as well as being subject to crevice corrosion.
Besides the need for crack arrestors, corrosion protection is desirable for pipes and other objects, even where no crack arrestors are involved, but to provide corrosion protection for a pipe in the region of a crack arrestor and to apply it in a manner which is compatable with the presence of crack arrestors presents additional problems.